Field of the Invention
The present invention relates generally to a power train control system for an internal combustion engine of a motor vehicle. More particularly, the invention is concerned with a power train control system which can fail-safe the operation of an internal combustion engine (hereinafter also referred to simply as the engine) and hence that of a motor vehicle, and which can support a backup operation of the engine and hence of the motor vehicle even in the event of occurrence of a failure in the operation of an electrically controlled throttle valve for regulating the intake air flow supplied to the engine as well as an electrical control system for controlling the throttle valve.
Description of the Related Art
The intake air flow fed to a gasoline engine is conventionally regulated by a throttle valve whose opening degree is controlled by means of an accelerator pedal mechanically interlocked to the throttle valve. In recent years, there is adopted in some practical applications a so-called wired power train control in which the opening of the throttle valve is controlled by an electrical actuator in accordance with an output signal of an accelerator pedal position sensor which signal represents depth of depression of an accelerator pedal of a motor vehicle, in an effort to enhance the comfortableness in driving the motor vehicle and realize a driving at a cruising speed, while improving a disposition layout of onboard attachments.
For a better understanding of the present invention, the background technique thereof will first be described in some detail.
FIG. 6 is a block diagram showing generally and schematically a structure of a power train control system for a motor vehicle known heretofore. Referring to the figure, an internal combustion engine denoted by a reference numeral 1 is equipped with an air intake pipe 1a in which a throttle valve 2 is installed for controlling or regulating the amount of air supplied to the engine. The throttle valve 2 is mechanically coupled to an electrical throttle actuator 3 which is constituted by a DC motor, a stepping motor or the like for actuating the throttle valve 2. To this end, the throttle valve 2 is connected to the throttle actuator 3 by means of a shaft 4. A return spring 5 is wound around the shaft 4 in such orientation that the throttle valve 2 is resiliently urged toward the closed position when operation of the throttle actuator 3 is disabled. Provided in association with the throttle valve 2 is a throttle position sensor (TPS) 6 for detecting the degree of opening of the throttle valve 2. On the other hand, there is provided in association with an accelerator pedal 7 of a motor vehicle an accelerator pedal position sensor (APS) 8 which serves for detecting the degree of actuation or depression of the accelerator pedal 7. A reference numeral 9 denotes an engine rotation speed sensor for detecting the rotation speed (rpm) of the engine 1 to thereby generate an engine rotation speed signal. The output signals of the sensors 6, 8 and 9 mentioned above are inputted to a controller 10 which is designed to control operation of the throttle actuator 3 on the basis of these sensor output signals.
With the power train control system of the structure described above, the control of the throttle actuator 3 is realized through a procedure which is illustrated in a flow chart of FIG. 7. Incidentally, processings and operations such as arithmetic operations, conditional decisions and others described below by reference to FIG. 7 are executed by a micro-computer incorporated in the controller 10. However, since such micro-computer is a conventional one, description thereof is omitted, being understood that the structure as well as programming of the micro-computer can easily be implemented by those having ordinary knowledge in the art.
Now, referring to FIG. 7, in a step S71, the controller 10 or micro-computer incorporated therein fetches the output signal of the accelerator pedal position sensor 8, which signal represents a degree of depression .alpha. of the accelerator pedal 7. In a step S72, a desired opening degree .theta..sub.s of the throttle valve 2 is arithmetically determined on the basis of the detected depression depth or actuation level .alpha. of the accelerator pedal 7. The arithmetic operation for determining the throttle opening .theta..sub.s can be performed in accordance with a function .theta..sub.s =f(.alpha., N.sub.e) (where N.sub.e represents the engine rotation speed) which has previously been established and stored in a memory incorporated in the controller 10. The relation between the accelerator pedal depression depth .alpha. and the desired throttle valve opening degree .theta..sub.s may differ in dependence on maneuvering performances or characteristics to be imparted to the motor vehicle. FIG. 8 graphically illustrates a typical one of such relations. Referring to this figure, a characteristic curve a in solid line indicates that the throttle opening degree .theta..sub.s is changed substantially linearly in proportion to the depression depth .alpha. of the accelerator pedal. In contrast, in the case represented by a characteristic curve b, the throttle valve opening degree .theta..sub.s is so controlled as to change gently in a range within which the accelerator pedal depression .alpha. remains small. With the maneuvering characteristic represented by the curve b, it is contemplated to cope with such problems that shock is likely to occur in the motor vehicle, making difficult the optimum control of the engine, when the intake air flow changes rapidly or steeply upon starting of the vehicle or in the course of running at a low speed.
On the other hand, FIG. 9 graphically shows relations between the engine output torque and the engine rotation speed (rpm). As can be seen from a broken-line curve labeled "ENGINE TORQUE", the output torque of the engine does not bear a linear relationship to the engine rotation speed (rpm). More specifically, in low- and high-speed ranges, the engine output torque tends to become low. In this conjunction, it is to be mentioned that the above-mentioned dependence of the engine output torque on the engine rotation speed ranges can be improved by correcting the relation represented by the curve b shown in FIG. 8 with correcting coefficients represented by a solid-line curve shown in FIG. 9 and labeled "CORRECTING COEFFICIENTS".
At this juncture, it should also be mentioned that the control characteristic of the throttle opening degree .theta..sub.s relative to the accelerator pedal depression .alpha. described above is only for the purpose of illustration. In reality, such control characteristics may vary in dependence on the desired maneuverability, comfortableness in driving the motor vehicle and/or other factors as well as performances of the engine.
After having determined the desired throttle opening degree .theta..sub.s as mentioned above, the processing proceeds to a step S73 (FIG. 7) in which a real or actual throttle opening degree .theta..sub.r is fetched from the output of the throttle position sensor 6, which is then followed by a step S74 where a deviation or difference e between the desired throttle opening degree .theta..sub.s and the real throttle opening degree .theta..sub.r is arithmetically determined. When the real throttle opening degree .theta..sub.r is smaller than the desired throttle opening .theta..sub.s, the throttle valve 2 is driven in the direction to increase the throttle opening .theta..sub.r on the basis of the deviation e through the throttle actuator 3 (step S75). If otherwise, the throttle valve 2 is driven in the direction to decrease the throttle opening .theta..sub.r through the throttle actuator 3 (step S76).
By controlling or regulating the opening degree of the throttle valve 2 through the electrical throttle actuator 3 in this manner, a high controllability of the engine operation and hence a high maneuverability of the motor vehicle can be realized. Besides, by feeding back a vehicle speed signal to the controller 10 to be taken into account in controlling the throttle valve 2, it is possible to effectuate a cruising-speed (constant speed) drive of the motor vehicle.
However, in contrast to the conventional mechanical control of the throttle valve in which the opening degree thereof is controlled by the accelerator pedal through the medium of a mechanical linkage, the electrical control of the throttle valve 2 described above is susceptible to a problem that the throttle valve 2 becomes inoperative, when a failure occurs in the throttle actuator 3, the controller 10 or other components taking parts in the electrical control of the throttle valve 2, which may lead to uncontrollable running of the motor vehicle. Accordingly, it is very important to fail-safe the electrical control of the throttle valve.
FIG. 10 is a flow chart for illustrating, by way of example, a procedure for making decision as to occurrence of abnormality in the control system for the throttle valve 2 inclusive of the throttle actuator 3 and measures taken in dependence on the results of the abnormality decision.
Referring to FIG. 10, in a step S101, there are determined the depression .alpha. of the accelerator pedal 7, the real opening degree .theta..sub.r of the throttle valve and the deviation or difference .beta. therebetween. The relation between the quantities .alpha. and .theta..sub.r can be given by a predetermined function, as described above. So long as the relation given by this function is maintained normal, there can not make appearance the difference .beta. of such magnitude which exceeds a predetermined value .beta..sub.1 (Step S102). Thus, it can be decided that the real throttle opening .theta..sub.r is abnormal when the above-mentioned difference .beta. exceeds the predetermined value .beta..sub.1.
When the difference or deviation .beta. is greater than the preset value .beta..sub.1, electric power supply to the throttle actuator 3 is interrupted (Step S103) to thereby stop the operation of the throttle actuator 3, because, if otherwise, there arises possibility of uncontrollable running of the motor vehicle due to the abnormality occurring in the throttle valve or in the control system therefor. When operation of the throttle actuator 3 is disabled, the throttle valve 2 is resiliently urged to move to the fully closed position under the effort of the return spring 5. On the other hand, when the difference .beta. is smaller than the predetermined value .beta..sub.1 inclusive thereof, it is decided that the throttle control system inclusive of the throttle actuator 3 is normal (step S104).
However, there may arise such situation that the throttle valve 2 is not moved to the fully closed position even when the operation of the throttle actuator 3 is stopped, because of a frictional engagement in a reduction gear train incorporated in the throttle actuator 3. To deal with this problem, it is known to dispose an electromagnetic clutch (not shown) between the shaft 4 of the throttle valve 2 and the throttle actuator 3 and disconnect the former from the latter by deenergizing the electromagnetic clutch upon occurrence of abnormality in the throttle valve control system to thereby allow the throttle valve 2 to assume the fully closed position under the effort of the return spring 5. Further, when a failure takes place in operation for opening the throttle valve 2, fuel injection to all or some of the engine cylinders may be interrupted to thereby lower the engine output torque. Alternatively, an ignition timing may be delayed to the substantially same effect.
On the other hand, when a failure occurs in the fully closed state of the throttle valve 2 (i.e., when the throttle valve 2 can not be opened from the fully closed position), the engine operation stops spontaneously to inhibit the motor vehicle from further running. As the measures for solving this problem, it can be conceived to mechanically couple the throttle valve 2 to the accelerator pedal 7 through manual operation or to provide throttle actuator systems in duplicate with one in redundancy.
As is apparent from the foregoing description, in the conventional power train control system for the motor vehicle, the throttle actuator control can certainly be fail-safed against failure in the fully opened position of the throttle valve (i.e., failure incurring uncontrollable running of the motor vehicle). However, for the failure occurring in the fully closed position of the throttle valve (i.e., failure disabling the running of the motor vehicle), there is no remedy method which can be adopted practically and profitably from the standpoint of economy and manipulation. It goes without saying that the disability of running the motor vehicle due to such failure will incur unwanted situation in dependence on the place where the motor vehicle is driven. For example, in the course of driving the motor vehicle on an expressway, this sort of failure should be excluded positively.